1. Field of the Invention
The present invention relates to an engine.
2. Description of the Related Art
The exhaust gas from an internal combustion engine can readily cause an exhaust pipe to heat to a high temperature. For this reason, a mechanism for cooling the exhaust pipe using cooling water is used. The outboard motor disclosed in Japanese Patent Laid-open No. 09-309497, for example, provides an engine and a heat exchanger. The engine includes a cooling water jacket for cooling the engine. The heat exchanger cools the heated cooling water circulating in the cooling water jacket. In this way, the starting performance of the engine can be improved more by cooling the engine indirectly than by cooling the engine directly. The heat exchanger cools the cooling water in the cooling water jacket by exchanging heat between the cooling water in the cooling water jacket and seawater from a periphery of the marine vessel. After the seawater cools the cooling water in the cooling water jacket at the heat exchanger, the seawater cools an exhaust system of the engine.
If the temperature of an exhaust pipe becomes too low this may impact the operation or performance of the engine. For example, in the outboard motor disclosed in Japanese Patent Laid-open No. 09-309497, the engine itself is indirectly cooled by the cooling water. However, the exhaust system is directly cooled by the seawater from the periphery of the marine vessel. Therefore, when the temperature of the seawater is low, the wall temperature of the exhaust pipe stays at an excessively low temperature. If the exhaust pipe stays at an excessively low temperature, condensation forms inside the exhaust pipe because water vapor in the exhausted gas that contacts with the inside surface of the exhaust pipe is rapidly cooled. When this happens, if a sensor or catalyst disposed in the exhaust pipe becomes moistened by the condensation, the performance of the sensor or the catalyst may deteriorate substantially. Moreover, if the temperature of the catalyst is not sufficiently high it may become difficult for the catalyst to be activated.